2023-06-09 13:25:53
Biomarker signatures dating back more than a billion years have revealed a whole community of previously unknown organisms that once shaped Earth’s ecosystems. Details of the study are published in the journal Nature.
Nowadays, cholesterol has a bad reputation, but remember that this lipid molecule is present in all animal cells and that it plays an essential role. These fats help in particular to maintain the fluidity and flexibility of the cell membrane. They also regulate the organization and assembly of membrane proteins, which is crucial for many cellular functions. Cholesterol also participates in the transport of lipids in the body. Finally, it is the main molecule from which other bioactive molecules are synthesized. These play an important role in the regulation of various physiological processes.
Note that cholesterol belongs to a family of similar molecules called sterols. Other eukaryotes also produce their own sterols, including stigmasterol in plants and ergosterol in fungi. Geochemists and paleontologists therefore sometimes turn to fossilized traces of these sterols to probe the presence of eukaryotes in ancient ecosystems. However, beyond 800 million years, the remains of these molecules seemed to be missing. Yet some fossil and genetic evidence suggests that eukaryotes had evolved long before that, 1.6 billion years ago. So how can this lack of data be explained?
In search of protosteroids
Several decades ago, biochemist and Nobel laureate Konrad Bloch proposed a potential explanation. According to him, in place of today’s sterols, earlier life forms could have used chemical intermediates in their cells. He named these compounds protosteroids. However, the technology available at the time was insufficient to prove their existence. Since then, techniques have evolved. So, in a new study, geochemists from the Australian National University went in search of the fossilized remains of some of these protosteroids.
First, the researchers synthesized protosteroids with barbaric names called lanostérol, cycloarténol et 24-methylene cycloartenol in laboratory. They then mimicked the fossilization process by exposing them to heat and pressure. In doing so, they then identified dozens of derivatives of these molecules likely to be distinguished other stages of cholesterol formation in the geological record.
The researchers then searched for these compounds in ancient rocks, which brings us back to this incredible discovery made in the Northern Territory of Australia. In the sediments formed there are 1.64 billion years old, they indeed identified chemicals corresponding to derivatives of lanosterol and cycloartenol. The researchers also found derivatives that matched the pattern produced by 24-methylene cycloartenol in rocks 1.3 billion years old.
A transition 800 million years ago
In younger rocks, formed between 800 and 720 million years ago, the team would have this time identified a “mixture of old and new”. In other words, there were significant amounts of fossilized protosteroidssuggesting that the organisms that depended on it had not yet died out, but also traces of modern cholesterol and sterols. The proportion of protosteroids would eventually have decreased over time in favor of modern sterols in rocks less than 600 million years old.
For the researchers, there would be a correlation between this evolution of fat molecules and the oxygen content at the time which was much lower than today. We know that the final stages of cholesterol synthesis are costly for cells. They therefore require a lot of energy and oxygen. Some eukaryotes would then have started modifying these compounds to make new sterols as this element became more abundant around 800 million years ago, giving their cells an evolutionary advantage. Subsequently, organisms that depended on protosterols disappeared.
Credits: MidJourney, by TA 2023
Although we do not know what these famous organisms might have looked like, we do know that they differed from complex eukaryotic life as we know it, such as animals, plants and algae in their cellular structure and probable metabolism. . We also know that they were considerably more complex than bacteria.
Trying to study these ancient worlds will be a huge challenge for scientists. Nevertheless, discoveries like this bring us closer to tracing our roots.
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